People say "cheap energy" though, and have something in mind. Sometimes they mean things like "our business model assumes cheaper energy than our competitors in country X."

IIRC Japan had a lot of business like that until 1973. All of the major industrial countries did, Japan perhaps the most. Except that although OPEC and the oil shock led to serious problems, the serious problems lasted only a year or so. "Can't function without cheap energy" was true? The dependency was solved so quickly that I find the word "can't" inappropriate.

Isn't a product necessarily an externality?

If they kept it internal, nobody else would have access to it.

No, an externality is when the transaction leaks costs/benefits into the wider society, outside of what's priced in.